v
e
e
Rubberlike materials, designated as Lactoprene, were prepared in earlier investigations b y copolymerizing ethyl acrylate with small proportions of butadiene, isoprene, or allyl maleate, compounding the resulting copolymers (assumed to have olefinic unsaturation) with sulfur and acceleraton, and then curing the compounded products. Since it was difficult to prevent cross linkage during polymerization of mixtures containing butadiene and other polyfunctional monomers, vulcanization of acrylic resins not having olefinic linkages was attempted. Poiyethyl acrylate and various saturated copolymers of ethyl acrylate were vulcanized satisfactorily with red
lead and quinone dioxime and also with benzoyl peroxide. The copolymers made from acrylonitrile, cyanoethyl acrylate, chloroethyl acrylate, chloropropyl acrylate, and phenyl acrylate were vulcanizable with certain sulfur-accelerator mixtures. The preparation of rubberlike materials b y vulcanizing saturated acrylic resins instead of copolymers of the ethyl acrylate-butadiene type has the following advantogor: (a) Agents and techniques to prevent cross linkage are not required] ( b ) the polymers and copolymers are soluble, and hence the viscosity of the solutions can be used as an index of the molecular weight) and (e) synthetic rubber cements can b e made.
w. c. MAST, c. E. kEHBERG, T. J. DIETZ, AND c. H. FISHER
7
HE vulcanization of copolymers (presumed to have olefinic unsaturation) made from alkyl acrylates and small proportions of butadiene, isoprene, allyl maleate, and similar polyfunctional monomers are described in other papers ( 4 , 9). Although the vulcanizates prepared in this manner were rubbery and seemed suitable as rubber replacements in some fields, polyfunctional monomers were generally objectionable because of their tendency t o create cross linkages prematurely-that is, during polymerization. After a study of the copolymerization of ethyl acrylate with many polyfunctional compounds had inc-iicated that cross linkage (or effects ordinarily attributed to it) nearly always occurs when this method is used to produce unaaturated copolymers, it was decided t o attempt the vulcanization of saturated acrylic resins. Although olefinic unsatuSation has generally been considered necessary for vulcanization ( 1O), it seemed likely that vulcanization could be effected through some combination of a nonolefinic functional group (ester, cyano, halogen, etc.) and a vulcanizing agent. Acrylic resins contain ester groups and one hydrogen alpha t o the carboxyl group that might enter into cross linkage or vulcanization reactions. Acrylic resins containing other functional groups were prepared by copolymerizing ethyl acrylate with small proportions of acrylonitrile, p-cyanoethyl acrylate, y-chloropropyl acrylate, and similar monomers. Vulcanization of these copolymers was attempted with various recipes including benzoyl peroxide and reinforcing agents, sulfur and organic accelerators, p-quinone dioxime and red lead, p-dinitrobenzene and litharge (IS),sulfur and litharge, and Polyac (14). The results of this study and certain properties of vulcanizates prepared from saturated acrylic resins are given in the present paper. The polymerizations were carried out as before (4,9) in roundbottom, threeneck, Pyrex flasks fitted with a thermometer well, reflux condenser, and water-sealed stirrer (ground-glass joints). Steam was passed through the emulsion to distill monomer or volatile impurities, and then coagulation was effected by the addition of a dilute solution of sodium chloride. The polymers were washed with water on a small washing mill and air-dried. The acrylic esters were emulsion-polymerized more successfully when proper consideration was given to purity of the monomer and the threshold or minimum catalyst concentration required for polymerization. The threshold catalyst concentration was related to the temperature, and only small amounts of catalyst (ammonium persulfate) were needed under refluxing conditions (approximately 82' C.). The monomers should he freshly
v
. . Eastern Regional Research Laboratory, Philadelphia, Pa.
distilled and, if not used immediately, stored so that the formation of peroxides is minimized. The removal of inhibitors by dilute sodium hydroxide should be followed by several washings and finally wltb with distilled water, dilute sulfuric acid (0.010/,), distilled water. Generally it is disadvantageous to use more than the threshold concentration of ammonium persulfate. When there is too much catalyst, refluxing may be so violent that the emulsion coagulates. When the catalyst concentration is below the threshold value, polymerization may not occur even after hours of refluxing. When carried out properly, polymerization proceeds smoothly a t refluxing temperature with little heating. A steam bath is more satisfactory than a water bath for this type of polymerization. The phenyl (a), chloropropyl, methoxyethyl, cyanoethyl, and nitroisobutyl acrylates were prepared in connection with other investigations (11). The compounding ingredients were milled into the polymers on a small rubber mill. The polymers usually were tacky and easily milled without the addition of plasticizers or softeners. Benzoyl peroxide (Luperco A) was so active as a vulcanizing agent that i t was difficult to prevent scorching on the mill, even when the peroxide was incorporated last. The compounded mixtures were cured in stainless-steel sandwich molds having the dimensions 4 X 4 X 0.032 inch or 6 X 6 X 0.075 inch. Cellophane sheets were used in the smaller mold. These were apparently detrimental, since in some instances the tensile strengths were lower. Unlike the copolymers ( 4 , 9) prepared with monomers having two or more olefinic linkages, most of the copolymers of the present work were soluble in organic solvents before vulcanization. The viscosities of solutions containing about 0.05 gram of polymer per 100 ml. of toluene were determined at 25' C. (constant-temperature bath) with modified Ostwald tubes. The natural logarithm of the relative viscosity divided by the concentration-that is, (In ~r/c)-wns used as an index of the average molecular weight of the polymers ( 6 ) . It was shown experimentally that the values for (In ?,/c) were approximately the same when c was 0.05 or extrapolated t o 0. VULCANIZATION OF POLYETHYL ACRYLATE
Polyethyl acrylate, prepared as shown in Table I, was compounded by several different recipes and molded at 298' F. (Table 11). The sulfur-Captax-Tuads combination gave unsatisfactory
1022
INDUSTRIAL AND ENGINEERING CHEMISTRY
N d r , 1944
1023
The ester group in polyethyl acrylate apparently is responsible f o r t h e vuic'anization withbenzoyl AmmoTerdtol nium Penperoxide and quinone dioxime, trant PerEthyl Expt. Acrylate, Copolymerising Monomer, No. 4a, Water, sulfat4 T t y , Time, Yield, since cross linkage did not occur Hr. Gram M1. Gram' Gronu No. Grama % e when polyisobutylene (Vistanex) 1.6 4 800 0.026 80-92 87.b 8.82 1 1bOml. None 3.96 waa compounded according t o 260 0.02 78-92 4 . 2 8 88.11 3.62 Acrylonitrile, 7 . 6 4 2 142.6 these two recipes and molded. 3.91 90.5 2.98 300 0.046 Acrylonitrile, 7 . 6 4 78-91 4.6 8 142.6 The mechanism of the vuIcaniza3.24 0.02 3200 81-90 1.26 4b 1470.0 y-Chloroprop 1 acr late 76 6.27 28 tion of polyethyl acrylate is not 300 0.016 78-92 1.6 60' 3.b3 5 142.6 8 - C h l o r o e t h y ~ a c r y ~ t ebe , ~ . 4 known, but earlier work by 82-91 2 4 300 0.016 88 I 136 8-Chloroethyl acrylate 16 0.12 78-9 1 1.78 8-Chloroethyl acrylatb, 6; 3 1bO 92 2.'43 7 89 Kharasch and Gladstone (7)with acrylonitrile. 6 2bO 0.03 2 91 3.80 4 78-92 8 142.6 Bensyl aorylate, 7 . 6 a peroxide and isobutyric acid is 1bO 0.1 0.83 90 3 76-92 0 96 Phenyl acrylate, 6 suggestive. They observed that 80-92 300 0.03 1 . 8 3 9 3 . 6 1nno1. LO 142.6 4 8-Methoxyethyl acr late 7 6 80-92 300 0.03 1.67 91 3.64 8-C anoqtbyl acrygte, i.b 4 11 142.6 isobutyric acid is converted into 300 0.02 1 . 6 7 91 4.66 2-~e-2-nitro-l-propy1 4 77-92 12 142.6 acrylate, 7 . 6 tetramethylsuccinic acid by treatalkyl sulfate. ment with acetyl peroxide.. Since * Sodium Triton 720 (8 grams) used: thia emulsifier is a sodium aslt of aryl alkyl polyether sulfonate (16). the polyacrylic ester chain is somewhat similar t o isobutyric acid in. having one hydrogen seeulta, but the Luperco A (benzoyl peroxide) and GMF (quialpha t o the carboxyl group, possibly cross linkage occurs none dioxime) recipes gave good vulcanizates. About 4 hours at through the same type of coupling. a98" F.was necessary for vulcanization with the quinone dioxime formula, and the vulcanizates thus prepared (Tables I1 and 111) VULCANIZATION OF ETHYL ACRYLATE COPOLYMERS had moderately high tensile strengths. Cross linkage occurred Ethyl acrylate was copolymerized with various monomers much more rapidly with benzoyl peroxide (10 to 20 minutes a t I), and the resulting CoPolYmers were compounded by 210" F.), but the products were relatively weak. [Cross-linked different recipes and molded t o ascertain whether the cyano, acrylic resins were produced &o by preparing ethyl acetab halogen, Phenyl, ether, and nitro groups in the copolymers solutions of the polymeric acrylic ester and benzoyl peroxide would facilitate vulcanization. The results show that several (Lucidol), applying the solution t o a surface, allowing the solvent functional groizps in the acrylate copolymers are susceptible t o to evaporate, and heating the resulting fdm at about 80" C. for cross linkage or vulcanization. It has been believed that olefinic B short time.] Table
I.
Preparation of Ethyl Acrylate
Table tl. Expt.
No. 1
Aorylonitrile, 6
3
Acrylonitrile, b
40
y-Chloropropyl acrylate, 4.8
6
8-Cbloroethyl acrylate, 5
Emulsion P o l y m r i u t i o n
Vulcanization of Polyethyl Acrylate and Ethyl Acrylate Copolymetp
Copolymeriaing Monomers, % None
2
Copolymers by
Compounding Recipe uinone dioxirnee ensoyl eroxided SulfurJ uinone dioximee enroyl peroxided Sulfur 0 uinone dioxime' enaoyl peroxided Sulfur%/ &oxime8
9 8 8 C&inin?e
Quinone dioximee Benioyl peroxided
Curing Time Tensile a t 298O F.. Strength M1n.b Lb./Sq. 1;. 240 1390 120 810
Elon Ultimate ation,
Shore A Hardneii
...
440
...
46
Tensile Product 710 365
240 180 240 240 120
1320 1000 850 1420 870
280 420 1040 340 620
72 13 60 70 62
345 420 860 480 460
160h
1610 1240 1610 870 1280 1350 1050 1220 1180 820
470 960 400 600 880 460 280 720 470 660
67 39 84 4b 46 66 60 42
766 1178 646 435 1126 620 295 880
..
...
. I .
...
210c 120 240 240 180 120 180 240 180
...
t% 610
56
..
...
..
...
...
Brittle Point,
c.
-- 16 O
16
...
-11 - 7 - 7
- Q - 8
...
... ...
- 1b
-
- 9 15 Quinone dioximeo 6 8-Chloroethyl acrylate, 10 16 Benaoyl peroxided -11 Bulfur. 14 Quinone dioximee 7 66 668 8-Chlp-ethyl aorylate, 6; acrylo- 6 Benaoyl peroxided zutrrle, 6 46 460 - 6 Sulfur%/ Quinone dioximee Benayl acrylate, 5 240 1410 B 480 85 676 -11 Benroyl peroxided 120 640 490 45 ais 10 Sulfur* >2400
